The present invention relates to a vented storage container and to a method for venting a container having an internal pressure that is greater than atmospheric pressure.
The storage and preservation of foods such as leavened dough prior to use by a consumer have posed problems because of dynamic properties,of the dough. Some of the problems occur because the dough is a substrate for dynamic chemical reactions that result in an evolution of carbon dioxide gas. The evolution of carbon dioxide gas causes an expansion of the dough, pressurizing the storage container. The rate of pressurization of the container is unpredictable because in part the rate and magnitude of carbon dioxide evolution during storage is dependent upon environmental factors such as temperature that are difficult to control. The rate and magnitude of carbon dioxide evolution are also dependent upon the process time as well as degree of work of the dough and formula of chemical reactants.
Other problems occur during storage of leavened dough because the dough is a substrate for oxidation reactions that undesirably form "grey dough." Gray dough occurs as a consequence of dough being exposed to oxygen for an excessive period of time. In particular, dough acquires a grey color when oxygen in a headspace of a container reacts with dough constituents during storage. It is believed that an oxygen concentration of as little as one to two percent oxygen within the container will result in grey dough.
Containers used to store foods such as leavened dough must be able to accommodate an internal pressure that changes over time and that is greater than atmospheric pressure. These containers should also be able to vent air from the can to prevent the formation of gray dough. Food storage containers that have been used for storing leavened dough have included composite cans. Composite cans include containers made of several layers of material and several types of material.
One type of composite can includes a cylindrical portion made of several material layers and two opposing circular ends attachable to the cylindrical portion to form a sealed can. During storage, the dough and evolved carbon dioxide exert a pressure against the can. The pressure is exerted against the cylindrical portion as well as the circular ends of the composite can.
For one type of composite can, the cylindrical portion includes an outer label layer, a middle paper layer and an impermeable inner liner. The cylindrical portion of one type of composite can also includes a spiral and overlapped seam on the outer label layer and a butt joint on the middle paper layer. The inner liner layer includes a spiral heat seal joint made by folding the inner liner layer back on itself to form an overlapped portion and sealing the overlapped portion.
The label layer is positioned so that the label spans the spiral butt joint of the middle paper layer. The label layer includes an inner surface that faces the middle paper layer and an outer surface. A coating of adhesive applied to the inner surface attaches the label layer to the middle paper layer.
To store a material such as leavened dough within the composite can, a first circular end having an inside surface is attached to one end of the cylindrical composite portion. The inside surface faces the cylindrical portion when installed in the cylindrical portion. The inside surface of the circular end includes an annular lip, a recessed rim adjacent to the annular lip and an annular shoulder adjacent to the recessed rim. The circular end is positioned on a rim of the cylindrical portion so that the rim of the cylindrical portion is positioned within the recessed rim of the circular end. The end is attached to the cylindrical portion by rolling the edge of the end to pinch the cylindrical portion and abutting the annular shoulder of the inside surface of the circular end to the liner of the cylindrical portion. A seamer is typically used to roll the annular lip to the circular end of the cylindrical portion thereby forming a seam.
Once the first circular end is sealed and secured to the cylindrical portion, the leavened dough is placed in the can and typically occupies less than the internal volume enclosed by the can. Volume not occupied by dough is a headspace of the can. The can is then sealed and secured at a second circular end opposing the first circular end, thereby trapping air in the headspace of the can.
The can remains sealed until opened by a user of the dough. The can is opened by the user peeling the label layer along the spiral seam thereby breaching the adhesive bond and weakening the can at the seam to a degree that causes the can to open along the spiral seam and butt joint. This mechanism for opening the can requires the internal pressure of the can to be greater than atmospheric pressure. In some can designs, the user may open a can by similarly removing the label and then pressing at the butt joint of the middle layer with a utensil such as a spoon.
In some composite cans that store dough, gases leak from the space enclosed by the can to the outside of the can in response to the increasing gas pressure from the leavening reactions in the dough enclosed within the can. However, no mechanism is deliberately provided within each can to accommodate or control leakage of the air entrapped in the can headspace. Thus, any leakage of gases from a can does not lend itself to adjustment. If oxygen in air entrapped in the can remains in contact with the dough for an extended period of time, this leads to product deterioration.
Gas leakage in a composite can may occur where the annular shoulder on the inside surface of one of the circular ends abuts the inner liner. In particular, gas leakage is believed to occur at a site where the heat seal joint of the liner faces the annular shoulder of the sealed circular end.
The liner includes a surface that is generally smooth. However, the heat seal joint on the liner layer perturbs the smooth liner surface and creates a site of an uneven thickness. The site may operate as a de facto passage to permit a release of gases from the interior of the can to the exterior of the can. For some composite cans, the heat seal joint creates tiny passages through which headspace gases within the can may pass to the outside of the can.
Because neither the designer of the can nor the manufacturer of the composite can incorporates a specific mechanism for permitting a can to release gases, the gas release of each can becomes haphazard at best. For some cans, the depth of the passage made by the heat seal joint is too shallow to permit a significant venting of gas. For other cans, the passage made by the heat seal joint is restricted when the circular end is sealed to the can. These composite cans release an insufficient quantity of gases. Thus, the consequent variable and inadequate venting causes product deterioration, variable pressures in the can and deterioration of the can.
Openings and passages that are too large allow the escape of gas from the inside of the can but also undesirably allow large quantities of fluids in the dough to contact and permeate the layers of the composite can. The contact deteriorates the can because the fluids can weaken the layers.